The consequences of earthquakes can be felt thousands of kilometers away, triggering a new set of quakes on unrelated fault lines. Evidence for this theory vindicates a growing group of geologists who argue that the Earth's tectonic plates operate as a more interconnected system than has been recognized.
Earthquakes produce different types of seismic waves, which spread through the crust around them. These are grouped as high frequency body waves and the more damaging low frequency surface waves.
In 1992 a magnitude 7.3 earthquake near the town of Landers, Southern California, generated surface waves that triggered other quakes in seismically active locations across the western United States, some 1,250 kilometers (780 miles) away. Recognition of these effects overthrew the previously widespread view that earthquakes produced aftershocks along the fault line on which they occurred, but did not affect other faults.
Twenty years later the largest strike-slip earthquake ever recorded, an 8.6 monster off the coast of Aceh, Indonesia, was followed within 50 hours by Japanese quakes of magnitude 5.5 and 5.7. In Science Advances Dr Kevin Chao of MIT argues that these events were connected, and that surface waves reached halfway across Asia, five times as far as the farthest quake triggered by the Landers event, to rattle the always-fragile plates beneath Japan.
"When the Indian Ocean earthquake occurred, the surface wave passed through the northeast of Japan, and the seismicity in the region was suddenly triggered," Chao said in a statement. When surface waves from elsewhere on the planet pass through a fault it can change the balance of forces within a fault. "If a fault with high stress is ready to fail, it will accumulate more stresses,” Chao added.
The Japanese fault line at the time was already stressed from the 2011 Tohoku earthquake, the source of the devastating tsunami. The paper reports that the two main quakes were part of earthquake clusters that migrated "from northeast to southwest at a rate of around 70 kilometers (43 miles) per day." The authors tracked the forces causing these quakes by measuring the coherence of seismic energy in boreholes across central Japan. Seismic energy usually increases in individual bore holes as a result of local factors, but a coherent rise across dispersed holes indicates regional tension.
“We find that seismic coherence begins increasing... immediately after the passing of seismic waves from the Indian Ocean Event,” the paper adds. The coherence lasted for 10 days, during which a swarm of small quakes were observed.
Coherence in central Japanese seismic boreholes, the vertical lines indicate the timing of the Aceh Earthquake and the two medium sized Japanese quakes. Credit: Dolery et al, Science Advances.
The authors acknowledge that there are several explanations for the surge of seismic activity, but all point back to a role for Aceh quake. “The overarching hypothesis supported by this study is that broad regions of Earth’s crust may be perturbed by dynamic stresses,” the paper argues, “Leading to cascading elastic effects where crustal material is forced into a metastable state, followed by a slow dynamical recovery to either the original or a new equilibrium state.”
Precise earthquake forecasting remains a distant goal but the latest work may help bring that ambition closer.